Radio frequency (RF) communication systems generally rely on the upconversion of a data signal to a suitable carrier frequency for transmission and then downconversion at the receiver. One popular architecture for wireless receivers is a superheterodyne design in which the frequency downconversion takes place in at least two stages. The received signal may first be downconverted from the carrier frequency to an intermediate frequency (IF). Downconversion to IF from the carrier frequency facilitates filtering and amplifying the signal by the front end of the receiver. Subsequently, the IF signal may then be downconverted to baseband to allow recovery of the data signal.
While IF receivers exhibit a number of desirable attributes, they typically require an image rejection strategy to compensate for the generation of the image frequency that results from the downconversion to IF. Receivers employing a quadrature architecture provide image rejection through the use of two distinct channels to form the in-phase (I) and the quadrature-phase (Q) components of the received signal. Through appropriate filtering and mixing of the I and Q components, image rejection of interfering jammers may be achieved. A quadrature generator mixes the sine and cosine components of the carrier signal to extract the I and Q components. As will be appreciated, if the two signal components have a phase relationship of 90° with respect to one another, the interfering image may be perfectly rejected.
In practice, the quadrature generator uses the signal output by a local oscillator that does not perform in an ideal manner. As a result, the generated I and Q signal components may have phases that are not exactly 90° with respect to each other or the signal components may experience different gains. Therefore, as used herein, the term I-Q imbalance includes phase mismatch, unequal gain, or both. Notably, any I-Q imbalance may result in imperfect cancellation of the interfering image. Consequently, many strategies have been implemented to correct for I-Q imbalance to improve image rejection.
Conventional approaches may involve applying a correction at IF to compensate for the I-Q imbalance. For example, FIG. 1 depicts a simplified block diagram of a prior art digital receiver 100. As shown, an RF signal is received at antenna 102 and fed through low noise amplifier (LNA) 104 to quadrature downconverter 106, which mixes cosine and sine signals at the carrier frequency from local oscillator 108 to generate the in-phase (I) and quadrature (Q) signal components. Each channel includes lowpass filters 110 and 112 and analog to digital converters (ADCs) 114 and 116, respectively, and provides the digitized components to digital signal processor (DSP) 120. Within DSP 120, the I and Q signal components are combined by adder 122, fed through filters 124 and input to I-Q correction block 126. The corrected signal is downconverted to baseband by IF rotation block 128 by mixing with output from a local oscillator (LO) 130, fed through filters 132, and gain adjusted by digital variable gain amplifier (DVGA) 134. Decimator 136 then reduces the sample rate of the signal and outputs to demodulator 138 to recover the data stream.
As will be appreciated, the I-Q correction provided by receiver 100 involves performing the calculations necessary to compensate for any I-Q imbalance to the signal at IF. Consequently, receiver 100 performs the correction at the digital sample rate of the IF signal and prior to gain adjustment by DVGA 134. Performing the I-Q imbalance correction in this manner may require considerable computational resources and power expenditure. Accordingly, the techniques of this disclosure are directed to providing I-Q imbalance correction with reduced power consumption and computational overhead.